UPPER MARLBORO, Md. — A scene from the airport of the future: A man's pulse races as he walks through a checkpoint. His quickened heart rate and heavier breathing set off an alarm. A machine senses his skin temperature jumping. Screeners move in to question him.

Signs of a terrorist? Or simply a passenger nervous about a cross-country flight?

It may seem Orwellian, but on Thursday, the Homeland Security Department showed off an early version of physiological screeners that could spot terrorists. The department's research division is years from using the machines in an airport or an office building — if they even work at all. But officials believe the idea could transform security by doing a bio scan to spot dangerous people.

Critics doubt such a system can work. The idea, they say, subjects innocent travelers to the intrusion of a medical exam.

The futuristic machinery works on the same theory as a polygraph, looking for sharp swings in body temperature, pulse and breathing that signal the kind of anxiety exuded by a would-be terrorist or criminal. Unlike a lie-detector test that wires subjects to sensors as they answer questions, the "Future Attribute Screening Technology" (FAST) scans people as they walk by a set of cameras. [Mark Godsey]